This invention relates to compensation of chromatic dispersion (hereinafter simply referred to as dispersion) encountered in optical waveguides.
In optical transmission systems, one of the ways of compensating for the dispersion produced by standard transmission fibre is to include in the transmission path one or more lengths of dispersion compensating fibre. This dispersion compensating fibre also exhibits dispersion, but the sign of the dispersion is the opposite of that exhibited by standard transmission fibre. Moreover the modulus of that dispersion is significantly larger, typically about four times larger, than that of standard transmission fibre, and in consequence the dispersion of long lengths of standard transmission fibre can be compensated by the use of significantly shorter lengths of dispersion compensating fibre. Amongst the drawbacks of this approach to dispersion compensation is the fact that dispersion compensating fibre is typically significantly more lossy and expensive than standard transmission fibre. Moreover its properties are generally considered not suitable for deployment of this fibre in the field, and so the effective span length is not the aggregate length, but only that of the standard transmission fibre.
An alternative approach to dispersion compensation is described in U.S. Pat. No. 4,953,939, this approach involving the use of a chirped Bragg reflection grating in a length of single mode optical fibre waveguide. Initially problems were encountered in the writing of long gratings section by section without unacceptably degrading the performance through the presence of stitch errors where individual sections fail to register quite correctly with their adjacent sections. However, with the advent of forms of active section alignment, such as described in U.S. Pat. No. 5,837,169, or in European Patent Application No. 0 878 721, it has been found feasible to create acceptable quality dispersion compensators employing chirped Bragg reflection gratings of between 2 and 3 metres in length. However these also exhibit significant loss. Typically, this loss may amount to between 5 and 11 dB, and amongst the factors contributing to this loss is the loss of the special fibre in which the Bragg reflection grating is formed, the loss incurred by the writing process employed for creating the grating, loss incurred by mode conversion by the grating into radiative cladding modes, loss arising from the fact that the grating is not fully saturated, and losses incurred by the propagation of the light twice through the circulator.
A primary object of the present invention is to provide a form of dispersion compensator of the circulator and Bragg reflection grating type that has reduced or eliminated loss.
According to a first aspect of the present invention there is provided a dispersion compensator having an optical amplifier optical pump optically coupled with one port of an optical circulator via an optical waveguide, which optical waveguide includes a chirped Bragg reflection grating and, between the grating and the circulator, a length of optically amplifying waveguide.
A superficial resemblance can be found between a dispersion compensator according to the present invention and the gain compensated optical amplifier of U.S. Pat. No. 5,636,301. Though both devices involve the use of circulators, optically amplifying waveguides and Bragg reflection gratings, the devices are in fact quite different devices. In particular the present invention is directed to dispersion compensation, whereas the gain compensated optical amplifier of U.S. Pat. No. 5,636,301 is not only not concerned with dispersion compensation, it is additionally incapable of functioning as a dispersion compensator. This is because it is specifically a device whose component Bragg reflection gratings have reflection wavebands that are spectrally separated by spectral guard bands, and accordingly any attempt to use the gain compensated optical amplifier of U.S. Pat. No. 5,636,301 for dispersion compensation of a signal would serve to punch spectral holes in that signal.
The optical coupling between the optical pump and the circulator may include a wavelength multiplexing coupler between the optical pump and the Bragg grating in order to divert any signal power not reflected by the Bragg grating away from entering the optical pump.
According to a second aspect of the present invention, there is provided a dispersion compensator having an optical amplifier optical pump optically coupled via a power splitter with two adjacent ports of a four-port optical circulator via respective optical waveguides optically in parallel, each of which optical waveguides includes a chirped Bragg reflection grating and, between the grating and the circulator, a length of optically amplifying waveguide. Alternatively, the power splitter may be dispensed with, and a separate pump used for pumping each of the optical amplifiers.
The amplification that is required in these dispersion compensators to offset their lossy components is typically significantly less than that typically required of optical amplifiers employed in transmission highways, and hence the pump power requirements, and consequential cost, are correspondingly smaller.
Other features and advantages of the invention will be readily apparent from the following description of preferred embodiments of the invention, from the drawings and from the claims.